The residual effects of hemorrhagic shock on pain reaction and c-fos expression in rats

To investigate the residual effects of hemorrhagic shock on pain reaction and c-fos expression, we performed formalin tests after hemorrhage and reinfusion in rats. Twenty adult male Sprague-Dawley rats were divided into Control (n = 10) and Postshock (n = 10) groups. The mean blood pressure of the Control group was 100-120 mm Hg, and that of the Postshock group was kept at 50-60 mm Hg for 30 min by draining blood. After 15 min of returning mean blood pressure to normal levels in the Postshock group, 10% formalin (3.7% formaldehyde solution, 100 microL) was injected into the left rear paw of both groups. Nociceptive behaviors were observed for 1 h after the formalin injection. The rats were killed at 2 h after the formalin injection, and the lumbar spinal cord was then stained for c-fos immunohistochemistry by using the avidin-biotin-peroxidase method. Animals in the Postshock group showed considerably less nociceptive behavior than those in the Control group. C-fos expression in the deep layer (IV-VI) of the spinal cord was significantly less in the Postshock group. In conclusion, decreases of nociceptive behaviors and c-fos expression were observed under normotensive conditions after hemorrhagic shock. The mechanisms governing these reactions remain unclear.

Formalin tests were performed after hemorrhage and reinfusion in rats. A stress-induced analgesia was observed under normotensive conditions after hemorrhagic shock. The mechanisms remain unclear.